In diffusers selectively distributing either warm or cool air, it is common practice to use different discharges for the warm and cool air. In perimeter zones in particular, it is desirable to have heat discharged towards the outside wall while cool air is directed into the space to cool the occupants. Thermoactuators, which are thermostatic devices containing a material which undergoes a reversible phase change with an associated expansion/contraction, are often used to achieve changeover. A typical phase changing material would be a wax-like material which is solid at the supply temperature of the cool air and is liquid at the supply temperature of the warm air. The expansion of the phase changing material in going from the solid to the liquid state provides the mechanical power to achieve changeover, and reset by spring bias upon a reverse phase change.